The present invention relates to a micro machine switch and a method for manufacturing thereof and in particular to a micro machine switch which allows the on/off control of a wide range of signal frequency, that is, from DC (direct current) signal frequency to signal frequency of one or more giga Hertz and a method for manufacturing thereof.
In the following the prior art will be described taking the case of the invention described in xe2x80x9cMicro Electromechanical Switchxe2x80x9d, by Yun Jason Yao, ROCKWELL INTERNATIONAL CORPORATION (Japanese Patent Laid-Open No. Hei9-17300).
FIGS. 16(a) and 16(b) are a plan view of the micro machine switch disclosed in Japanese Patent Laid-Open Hei9-17300 and a cross-sectional view of the same taken along the line D-Dxe2x80x2, respectively. As shown in the same figures, an anchor structure 52 comprising thermosetting polyimide, a lower electrode 53 composed of gold and signal lines 54 comprising gold are provided on a substrate 51 comprising gallium arsenide.
And on the anchor structure 52 provided is a cantilever arm 55 consisting of silicon oxide, which extends to the signal lines 54 across the lower electrode 53 and faces the same with a space left between them.
An upper electrode 56 comprising aluminium is formed on the top of the cantilever arm 55 in such a position so as to face the anchor structure 52 and the lower electrode 53. And a contact electrode 57 comprising gold is provided on the bottom of the cantilever arm 55 in such a position so as to face the signal line 54.
In the micro machine switch having such construction, when applying a voltage of 30 V between the upper electrode 56 and the lower electrode 53, the force of attraction is applied to the upper electrode 56 in the direction of the substrate (downward in the direction shown by the arrow 58 by the electrostatic force). Therefore, the cantilever arm 55 is strained toward the substrate side; as a result, the contact electrode 57 comes in contact with both ends of the signal lines 54.
In the normal state, a space is provided between the contact electrode 57 and the signal lines 54, as shown in FIG. 16(b); accordingly, the two signal lines 54 are separated from each other. Thus, in state where no voltage is applied to the lower electrode 53, no current flows through the signal lines 54.
On the other hand, in state where voltage is applied to the lower electrode 53 and the contact electrode 57 is in contact with the signal lines 54, the two signal lines 54 short-circuit, which allows current to flow between them. Thus, the application of voltage to the lower electrode 53 allows the on/off control of the electric current or signals passing through the signal lines 54.
However, in order to reduce the switch loss particularly when using the switch for the signals in the microwave range, it is important that the upper electrode 56 and the contact electrode 57 are well insulated from each other. In other words, if the upper electrode 56 and the contact electrode 57 short-circuit, signals (including DC) flowing through the signal lines 54 flow out even to the upper electrode 56.
Even if the upper electrode 56 and the contact electrode 57 do not short-circuit, in state where electrostatic capacity is significantly large, alternate signals flowing through the signal lines 54 also flow out to the upper electrode 56 and leak outside.
As described above, when the upper electrode 56 and the contact electrode 57 are not well insulated, the leak of signals becomes large, and switching characteristics deteriorate. From this viewpoint, the prior art described above uses an insulating material (silicon oxide) as the material constituting the cantilever arm 55.
The micro machine switch of the prior art described above has the following problems.
The cantilever arm 55 is contact with the upper electrode 56 and the anchor structure. 52, both of which differ from the cantilever arm 55 in the material, over a wide range. Further, the cantilever arm 55 is designed to have a mechanically flexible construction so as to control the driving voltage of the switch and to move only by applying micro voltage.
As described above, since the upper electrode 56, the cantilever arm 55 and the anchor structure 52 are formed of different materials, their thermal expansion coefficients are also different, and particularly in the cantilever arm 55, warps are likely to be caused due to distortion.
For example, when comparing silicon dioxide, aluminium and polyimide, the thermal expansion coefficient of silicon dioxide is about 1/100 times as small as those of the other two. Therefore, the metal portion of the upper electrode 56 etc. expands with the changes in the processing temperature and the atmospheric temperature after the completion of a device, and thereby warps are easily caused in the cantilever arm 55.
The existence of such warps has a bad influence on switching characteristics regardless of their direction relative to the substrate 51. In cases where the cantilever arm 55 warps upward, even if the bottom side surface of the cantilever arm 55 comes in contact with the lower electrode 53 by the application of voltage, a state is likely to occur in which the contact electrode 57 does not come in contact with the signal lines 54. In that case, even if the contact electrode 57 comes in contact with the signal lines 54, the intensity of pressure applied to the contact portion is very small, and such a very light contact gives rise to a problem of increasing the contact resistance of the switch.
On the other hand, in cases where the cantilever arm 55 warps downward, although the contact electrode 57 surely comes in contact with the signal lines 54 due to application of a voltage, the entire contact electrode does not come in planar contact with the signal lines 54, and what is called single contact (both come in contact with each other only at one area) is very likely to occur. Thus, even in this case, there arises a problem of increasing the contact resistance of the switch.
As described above, in any case, a problem is created such that warps caused in the cantilever arm 55 increase the contact resistance, resulting in increasing the switch resistance when the switch is in the on state.
In fact, in the micro machine switch according to the prior art, the switch production process is performed at a low temperature of 250xc2x0 C. or lower, and thereby warps caused due to the processing temperature are controlled.
To be concrete, a silicon dioxide film for making up the cantilever arm 55 is formed by the plasma enhanced CVD (PECVD) process. A PECVD oxide film offers the advantage of being able to be formed at low temperatures, and keeping the processing temperature low is important when decreasing the influence of the big difference in thermal expansion coefficient from material to material.
On the other hand, it is well known that mechanical properties (distortion, rigidity, reliability, etc.) and electrical properties (dielectric constant, maximum breakdown voltage, etc.) of materials can be remarkably improved by the optimization of, particularly, temperature conditions.
However, in the micro machine switch according to the prior art as described above, since the processing temperature needs to be kept low, the temperature parameter cannot be made good use of for optimizing the materials. In this respect, the prior art can be said to be limited largely in material.
Generally, a cantilever arm has the advantage that the width of its arm can be decreased when increasing its thickness so as to keep the rigidity constant. Thus, it has another advantage that the dimensions of the entire switch can be decreased, therefore, multiple switches can be formed in a limited area.
However, in the micro machine switch according to the prior art, which utilizes silicon dioxide for the cantilever arm, it is limited largely in the thickness of the cantilever arm 55. In principle, the thickness of silicon dioxide film can be increased to 10 xcexcm or more by increasing the PECVD duration; however, increasing the growth duration means decreasing the processing rate of the PECVD system, which results in increasing costs and giving rise to various problems involving the maintenance of the system, for example, such that cleaning is often required since dust is likely to be caused.
Further, inside the thick film a large amount of distortion is caused, which gives rise to a problem of fracturing the substrate 51 during the deposition of silicon dioxide. For the reasons as above, the actual situation is such that the thickness of the cantilever arm is limited to about 2 xcexcm at most. Thus the micro machine switch according to the prior art is strictly limited in design dimensions of its construction.
In addition, the decrease in processing temperature as is performed in the prior art is effective to some extent in controlling warps caused during the manufacturing process, but not effective at all in controlling warps caused due to the variation in atmospheric temperature under which the micro machine is used. The problem of causing warps during the use of the micro machine is an inevitable consequence of using laminated films different in thermal expansion coefficient in the arm portion of the switch.
At the same time, the micro machine switch having the construction according to the prior art has the problems of mechanical strength and durability. When driving the switch, the highest stress is set up at the base of the cantilever arm 55 (at the portion where the cantilever arm 55 is connected to the anchor structure 52). Accordingly, in order to improve the mechanical strength and durability of the switch, it is necessary to optimize the structure of the base portion.
In the construction of the micro machine switch according to the prior art, the cantilever arm 55 and the anchor structure 52 are formed of different materials and placed perpendicularly to each other. And this construction is not suitable for relieving the stress given rise to at the base portion.
The present invention has been made so as to overcome the above problems, accordingly the object of the present invention is to provide a micro machine switch which can be mass-produced, is low-cost and is high-performance and a method for manufacturing thereof
In order to achieve the above object, one aspect of the micro machine switch according to the present invention relates to a micro machine switch for controlling the conduction/non-conduction between first and second signal lines which are provided on a substrate in such a manner as to leave a predetermined gap between their respective end portions. The micro machine switch as above includes: a supporting member provided on the above substrate in such a manner as to be adjacent to the above gap and to have a predetermined height with respect to the surface of the above substrate; a flexible beam member provided in such a manner as to project from the supporting member almost horizontally to the above substrate surface and to extend so far that its part faces the above gap; a contact electrode provided on the substrate side of the above beam member in such a position as to face the above gap, and a lower electrode provided on the above substrate in such a manner as to face part of the above beam member. Part of the above beam member, which ranges from the portion at which it is connected to the above supporting member to the portion facing the above lower electrode, is allowed to be conductive, and hence to function as an upper electrode. And the thermal expansion coefficients on the substrate side of the above beam member and on the opposite side of the same are allowed to be almost symmetric in the thickness direction perpendicular to the above substrate surface, at least in the region ranging from the portion at which it is connected to the above supporting member to the portion facing the above lower electrode.
In another aspect of the micro machine switch according to the present invention, the angle made between the substrate side surface of the above beam member and the surface of the above supporting member to which the above beam member is connected is an obtuse angle.
In another aspect of the micro machine switch according to the present invention, the above supporting member protrudes higher than the surface of the above beam member opposite to the substrate side surface of the same, at the portion both members are connected to each other.
In another aspect of the micro machine switch according to the present invention, the angle made between the beam member surface opposite to the substrate side surface of the same and the surface of the above supporting member protruding higher than the above beam member surface is an obtuse angle.
In another aspect of the micro machine switch according to the present invention, the angle made between the substrate side surface of the above beam member and the side surface of the above supporting member to which the above beam member is connected is an obtuse angle.
In another aspect of the micro machine switch according to the present invention, the above contact electrode is provided on the above substrate side of the above beam member via an insulating member.
Another aspect of the micro machine switch according to the present invention is characterized in that on the surface of the above beam member, which is opposite to the surface provided with the above contact electrode, a reinforcing member is provided in such a manner as to face the above contact electrode.
In another aspect of the micro machine switch according to the present invention, the above contact electrode is coated with an insulating material film capable of capacity connecting to the above first and second signal lines.
In another aspect of the micro machine switch according to the present invention, the above lower electrode is provided on the above substrate in such a position so as to be between the above supporting member and the above gap.
In another aspect of the micro machine switch according to the present invention, the above supporting member and at least part of the above beam member are formed of the same conductive material as an integral unit.
In another aspect of the micro machine switch according to the present invention, part of the above beam member, which ranges from the portion at which it is connected to the above supporting member at least to the portion facing the above lower electrode, is formed of a conductive member and on the tip portion of the conductive member an insulating member is provided which extends to the position facing the above gap, the above contact electrode being provided on the insulating member in such a manner as to face the above gap.
In another aspect of the micro machine switch according to the present invention, the above conductive member consists of a semiconductor material.
In another aspect of the micro machine switch according to the present invention, the above beam member consists of a semiconductor material and at least the region ranging from the portion at which the above contact electrode is provided to the portion facing the above lower electrode is insulated.
In another aspect of the micro machine switch according to the present invention, the above semiconductor material is a single crystal semiconductor.
In another aspect of the micro machine switch according to the present invention, the above semiconductor material is an amorphous semiconductor or polycrystalline semiconductor.
In another aspect of the micro machine switch according to the present invention, the above substrate is a glass substrate or a ceramic substrate.
In another aspect of the micro machine switch according to the present invention, the above substrate is a gallium arsenide substrate.
Another aspect of the micro machine switch according to the present invention is used in a phased array antenna system.
One aspect of the method for manufacturing a micro machine switch according to the present invention relates to a method for manufacturing a micro machine switch for controlling the conduction/non-conduction between first and second signal lines which are provided on a substrate in such a manner as to leave a predetermined gap between their respective end portions. The method includes: a step of forming a lower electrode on the above substrate; and a step of bonding a member, which consists of a supporting member having a predetermined height, a flexible beam member provided on the supporting member and a contact electrode provided on the beam member, to the above substrate while allowing the above contact electrode to face the above gap and to be spaced apart from the first and second signal lines. The above beam member is formed in such a manner that its part, which ranges from the portion at which it is connected to the above supporting member to the portion facing the above lower electrode, is allowed to be conductive and hence to function as an upper electrode and that the thermal expansion coefficients of its both sides, at least the region ranging from the portion at which it is connected to the above supporting member to near the portion facing the above lower electrode, are allowed to be almost symmetric in the thickness direction perpendicular to the above substrate surface.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the angle between the substrate side surface of the above beam member and the surface of the above supporting member to which the above beam member is connected is made an obtuse angle.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above supporting member is formed in such a manner as to protrude higher than the surface of the above beam member opposite to the substrate side surface of the same, at the portion both members are connected o each other.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the angle between the beam member surface opposite to the substrate side surface of the same and the surface of the above supporting member protruding higher than the above beam member surface is made an obtuse angle.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the angle between the substrate side surface of the above beam member and the side surface of the above supporting member to which the above beam member is connected is made an obtuse angle.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above contact electrode is provided on the above substrate side of the above beam member via an insulating member.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, on the surface of the above beam member opposite to the surface of the same which is provided with the above contact electrode, a reinforcing member is provided in such a manner as to face the above contact electrode.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above contact electrode is coated with an insulating material film capable of capacity connecting to the above first and second signal lines.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above lower electrode is provided on the above substrate in such a position so as to be between the above supporting member and the above gap.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above supporting member and at least part of the above beam member are formed of the same conductive material as an integral unit.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, part of the above beam member, from the portion at which it is connected to the above supporting member at least to the portion facing the above lower electrode, is formed of a conductive member and on the tip portion of the conductive member an insulating member is provided which extends to the position facing the above gap, the above contact electrode being provided on the insulating member in such a manner as to face the above gap.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above conductive member is formed of a semiconductor material.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above beam member is formed of a semiconductor material and at least the region ranging from the portion at which the above contact electrode is provided to the portion facing the above lower electrode is insulated.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, a single crystal semiconductor is used as the above semiconductor material.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, an amorphous semiconductor or polycrystalline semiconductor is used as the above semiconductor material.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, a glass substrate or a ceramic substrate is used as the above substrate.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, a gallium arsenide substrate is used as the above substrate.
In another aspect of the method for manufacturing a micro machine switch according to the present invention, the above micro machine switch is used in a phased array antenna system.
In the micro machine switch of the present invention constructed as described above, thermal expansion coefficients of the both side of the beam member material are almost symmetric in the thickness direction perpendicular to the substrate surface. Accordingly, warps caused due to the distortion occurring between different materials, as seen in the micro machine switch of the prior art, are considerably relieved. The simplest method for allowing the thermal expansion coefficients to be symmetric in the thickness direction is to form the beam member of only one material. It goes without saying that the beam member can also be formed to have a laminated structure which is symmetric in the vertical direction.
In order to suppress warps caused in the beam member, it is effective to allow the thermal expansion coefficients to be symmetric as described above particularly in the neighborhood of the portion at which the beam member is connected to the supporting member, more concretely, in the region ranging from the portion at which the beam member is connected to the supporting member at least to the portion facing the lower electrode. Conversely, in the neighborhood of the tip portion of the beam member, even if the thermal expansion coefficients are not symmetric in the thickness direction, warps are not very large.
When making measurements for the experimentally manufactured switches, it was found that the variation in contact resistance created due to single contact etc. was narrowed and thereby a large amount of switches uniform in characteristics could be manufactured.
It was also found that the change in switching operation was considerably small even if the ambient temperature of the switch changed.
Further, the micro machine switch according to the construction of the present invention has been improved in mechanical strength, durability and operational speed, compared with that of the prior art. For example, allowing the angle between the beam member and the supporting member on the substrate side to be an obtuse angle makes it possible to prevent the base of the beam member from being fractured by a stress concentration. Allowing the supporting member to protrude higher than the beam member makes it possible to give the structure in the neighborhood of the base of the beam member a shape which is almost symmetric in the vertical direction. This allows the thermal expansion coefficient distribution in the neighborhood of the base of the beam member, including the supporting member, to be symmetric, which is effective in preventing warps caused in the beam member.
The upward protrusion of the supporting member effectively improves the operational speed of the switch. Specifically, it is effective in increasing the speed of the switch""s returning motion, from the on state (the state in which the switch is dropped) to the off state (the state in which the switch is raised). This is because stresses set up at the base of the beam member become larger when the supporting member protrudes higher than the beam member. When turning on/off the switch, since the operation is very rapid, a phenomenon can sometimes occur that the beam member vibrates up and down (referred to as bound chattering). In order to stop this bound chattering promptly, it is necessary to let the supporting member absorb the kinetic energy of the beam member by setting up a moderate stress at the base of the beam member, and for this purpose, the structure is effective in which the supporting member protrudes higher than the beam member.
In the micro machine switch as above, the beam member is weakest of all the members in terms of the mechanical strength. Accordingly, the micro machine switch preferably has a structure which allows preventing the beam member from fracturing due to the contact with the substrate etc. when mounting the same on the substrate in its manufacturing process. Thus, if the micro machine switch is constructed in such a manner that the supporting member protrudes higher than the beam member, the contact accidents of the beam member can be suppressed, and thereby the likelihood of fracturing the switch can be reduced.
And when protruding the supporting member higher than the beam member, preferably the angle between the surface of the beam member and that of the supporting member on the opposite side to the substrate is allowed to be an obtuse angle, because doing so allows preventing the base of the beam member from fracturing due to a stress concentration. When the angles between the beam member and the supporting member are allowed to be obtuse angles both on the substrate side and on the opposite side to the substrate, both of the angles are preferably in the range of 100xc2x0 to 170xc2x0, more preferably 110xc2x0 to 150xc2x0 This allows the effect of decreasing a stress concentration as described above and the effect of setting up an appropriate stress to improve the operational speed to be compatible with each other.
Forming the supporting member and part of the beam member including at least the base portion of the same material allows reducing of the warps caused between the two members, suppressing of a stress concentration on one point and improving the strength of the micro machine switch, which results in improving the durability of the micro machine switch. And if the supporting member, the beam member and the upper electrode are all formed of the same material, the manufacturing process can be simplified.
Further, since the high-temperature process can be utilized, the number of alternatives for the materials constituting the beam member etc. is increased and various types conductors and semiconductors can be utilized, thus the degree of freedom of selecting material is increased. The insulating film formed at high temperatures in particular is excellent in resistance to pressure, this may contribute to electrical characteristics of devices.
Further, since the degree of freedom of the beam member (cantilever arm) is increased in the thickness direction, the width of the arm can be decreased, and thereby the size of the switch can also be decreased.
The beam member constituting the micro machine switch of the present invention is conductive at least for the part ranging from the portion at which it is connected to the supporting member to the position facing the lower electrode, and the xe2x80x9cconductivexe2x80x9d materials herein used are not limited to conductors such as metals. The point is whether or not voltage can be applied through the supporting member to such a position of the beam member so as to face the lower electrode, and current hardly flow this portion. Accordingly, as the materials for use in the part of the beam member ranging from the portion at which it is connected to the supporting member to such a position so as to face the lower electrode, a wide variety of materials such as metals-and semiconductors can be adopted. And when using semiconductor materials, the presence and the concentration of impurities added can be changed widely.
Because of the excellent effects as described above, the micro machine switch of the present invention can be applied not only to a simple switch which uses only a single micro machine switch, but also to a phased array antenna system which requires micro machine switches of order of tens of thousands to be integrated on a large area of substrate.